In some places the water supply has so much hardness such as calcium, magnesium, and other dissolved minerals that when the water is heated, solid precipitates form and accumulate as scale inside the water heater and hot water pipes. Scale accumulation inside the water heater typically reduces the life of large tanks, such as tanks used by office buildings and apartment buildings with central hot water systems. Domestically, scale buildup is not as much of a concern; household water heaters and pipes last 10-20 years before scale becomes a problem, even when the water supply is very hard. However, many households still purchase water softeners. In areas where the water supply is hard, apartment complexes with central hot water supplies need to de-scale the hot water pipes about once every 2-6 years, currently costing up to $30,000 depending on the amount of damage to the pipes. Typically, iron pipes can be de-scaled about three times before they must be replaced.
A secondary problem is scale buildup in the water heater. One water heater manufacturer offers a “turbo” model which uses small orifices on the input pipe to create turbulence inside the tank. The concept is to flush out scale buildup with the turbulence. Unfortunately, it is cheaper to keep the scale in the water heater; a commercial size domestic water heater costs about $4,000 (including labor) to replace, while de-scaling the pipes is much more expensive.
Current water treatment methods are too effective and therefore too costly for applications where the objective is reducing scale buildup in the water heater and hot water pipes. Ion exchange water softening systems, such as supplied by Culligan, are both expensive to install and costly to maintain. These systems require regular maintenance and when used to treat large quantities of water the upkeep makes them economically unfeasible. Also, ion exchange systems pollute the sewer with brine. The following discussion regarding ion exchange water softening systems is substantially based upon a discussion published by Culligan:
Ion exchange water softening systems normally address only bicarbonate hardness or, if more complicated, the total metal and salt content of water. However, these systems need regular maintenance such as the regeneration of the ion-exchange resin. If such maintenance is not carried out, these systems can actually produce treated water of worse quality than untreated water. Chlorine can damage the ion-exchange resins in these systems. Moreover, microbiological contaminants may be significantly increased due to microbiological growth on the resin if chlorine-free water is being treated.
Calcium and magnesium ions can combine with other ions and compounds to leave a hard scale on the surfaces they touch so ion exchange water softening systems reduce hardness problems by exchanging calcium and magnesium ions with sodium ions. Sodium ions are considered “soft” ions because sodium ions normally remain dissolved in the water and do not precipitate to form a hard buildup like scale.
A typical water softener has a pressure tank partially filled with ion exchange resin. For example, Culligan's brand of resin, Cullex® resin, consists of highly porous, amber colored, plastic beads loaded with “exchange sites” that remove hardness ions and replace them with sodium ions. A softener system also includes a brine tank to provide a source of sodium ions (salt) for regenerating the resin and hydraulic controls to direct the flow of water through the softener during service and regeneration.
At the beginning of the softening cycle, sodium ions occupy the resin's exchange sites. As water passes through it, the resin's stronger attraction for the hardness ions causes it to take on the hardness ions and give up its sodium ions. Iron and manganese are considered hardness and they are removed also, provided they are in solution. Ion exchange cannot remove suspended matter.
As water flows downward through the resin bed, the resin at the top of the bed gives up its sodium first. The exchange process is not instantaneous, so exchange occurs in a band called a “reaction zone”. The reaction zone's depth depends on incoming water hardness and total dissolved solids (TDS), flow rate, water temperature and resin particle size. When the reaction zone's leading edge reaches the bottom of the resin bed hardness is passed into the service line, the resin has become “exhausted” and it must be regenerated before it can remove hardness again.
The regeneration cycle starts with backwash, an upward flow that loosens the resin bed and flushes out suspended particles. Backwash may last about 10 minutes. Regeneration occurs when a solution of sodium chloride (salt) brine is passed through the resin in a downward direction. An educator draws concentrated brine from a storage tank and dilutes it to the right concentration. Brine draws lasts from 10 to 30 minutes depending on salt dosage (weight of salt per volume of resin). A large excess of sodium ions causes the resin to release its hold on hardness ions picked up during the preceding service cycle and returns the resin to its sodium state.
The brining step is followed by a slow downflow rinse to displace spent brine from the resin. It also carries the hardness removed from the resin to drain. The rinse rate is regulated to ensure correct contact time between the salt and the resin. Slow rinse usually lasts about 30 minutes.
A final fast downflow rinse, or purge, flushes all remaining brine from the tank. It lasts about 5 minutes.
Each cubic foot of Cullex® resin can remove about 30,000 grains of hardness when dosed with 15 pounds of salt (one liter of resin can remove 69 grams of hardness when regenerated with 240 grams of salt). A lower dosage of 6 pounds per cubic foot yields about 20,000 grains of capacity (46 grams capacity at 96 grams of salt per liter). This lower dosage is more salt efficient, but it requires more frequent regeneration. Most softeners are selected on the basis of a balance between capital cost and operating cost.
Another treatment technique is reverse osmosis. Reverse osmosis systems can treat large quantities of water, as in semiconductor factories, and purify water through a reverse osmotic membrane. In addition to filtering out the carbonates that cause scale, these systems also remove heavy metals and biological contaminants. Reverse osmosis systems are more expensive than ion exchange systems and are directed towards improving water quality rather than only removing the chemicals that become scale. Reverse osmosis systems require significant maintenance. Moreover, volatile organic compounds (VOC) and trihalomethanes (THMs) are not treated and these together with chlorine, can actually damage the reverse osmotic membrane and reduce its effectiveness.
Reverse osmosis (RO) is a separation process that uses pressure to force a solution through a membrane that retains the solute on one side and allows the pure solvent to pass to the other side. More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. The membrane here is semi-permeable, meaning it allows the passage of solvent but not of solute.
The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2-17 bar (30-250 psi) for fresh and brackish water, and 40-70 bar (600-1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome.
Both ion exchange and reverse osmosis systems remove scale, but at a cost where it is cheaper for office buildings and apartment complexes to de-scale and replace hot water pipes than purchase and maintain a water softener.